1. Technical Field
The present invention generally relates to a terminal polling method and, more particularly, to a method of terminal polling in which the effect of interference between terminals which responds at substantially the same time is reduced.
2. Description of the Relevant Art
Two-way interactive cable television systems are known for transmitting entertainment, information and data signals over a cable facility to a plurality of users. Data may be transmitted and addressed to a particular subscriber over a separate data channel or a so-called "in-band" data channel. In a downstream direction, addressed control data may represent services authorized to a particular terminal or control commands to that terminal. In an upstream direction from a terminal to the service provider or system manager location, control data may represent selections made by a user in response to a polling request or at the time of user selection.
In systems where many transmitters supply information to one receiving point, it is important to control transmissions so as to minimize the effects of mutual interference or collisions between a large number of transmitters. A number of prior art techniques have been developed to control such mutual interference. For example, it may be possible to pre-order transmissions from a plurality of transmitters such that each transmitter transmits at a unique, predetermined time after receipt of a command which requires a response. However, this method is inefficient when not all transmitters are required to respond or when transmitters are frequently added to or removed from a system.
A number of other prior art techniques are set forth in U.S. Pat. No. 4,409,592 to Hunt. In the slotted Aloha protocol, for example, collision effects are reduced and channel utilization increased by partitioning the common radio channel into slots of time equal to a packet length and having each station only transmit a packet at the beginning of a slot. In this way, overlapping transmissions are forced to completely overlap and channel utilization is thereby increased.
In carrier sense schemes, any carrier signal on the communication channel must be sensed by a station before it transmits a packet and if a carrier is detected, the transmission of the packet is deferred until the end of the carrier signal. This concept is well understood and has been used in voice aircraft radio communication.
Finally, collision detection schemes are based on the assumption that a station in the act of transmission can detect transmissions from other stations and abort transmission if an overlap is detected.